SEO is a crucial part of the success of any online enterprise. But it's important to remember that the practice is constantly changing and evolving, so many of the techniques previously sworn by in previous years are totally defunct since Google et al have rolled out numerous high-impact algorithm updates. A lot of SEO agencies and freelancers aren't prepared to keep up with all these changes and so their work will be less effective (and often even detrimental to your promotional campaign). Invest in the right SEO approach and the sky really is the limit.